This invention relates to wireless communication and in particular to environments characterized by multipath interference. More particularly, this invention relates to improvements over conventional adaptive array processing of signals in the presence of multipath.
Traditional approaches to adaptive processing of signals received through an antenna array are typically constrained by the number of antenna elements. For example, the number of beams or nulls formed cannot exceed the number of elements. The components required to derive the amplitude and phase information for array processing increase proportionately with the increase in the number of antenna elements. In addition many conventional adaptive array processing algorithms use matrix inversion and manipulation which requires expensive high power processing. Traditionally, the direction of the intended user (IU) or source of interference to be canceled must be acquired before a beam or null can be steered in that direction. This requires either complex processing over time (which may not account for the system dynamics) or relaxing the benefits of beamforming by using wider beams or sweeping a narrow beam for acquisition. Invariably such acquisition necessitates a difficult compromise between performance and complexity (i.e. cost).
Beamforming and nulling approaches suffer inherent potential performance reductions. If a null is steered in a particular direction, for example, all energy in that direction is normally canceled in conventional approaches to adaptive array processing. Portions of the intended user signal could thus be lost if it comes from such a direction. (One scenario where this can be easily seen is the case where the multipath from the intended user signal source reflects from an object in the same general direction as the intended userxe2x80x94a very likely scenario in an urban environmentxe2x80x94making it impossible to null the multipath signal without impacting the main intended user signal). Even if the intended user signal does not come from the same direction as the interference, the sidelobes and backlobes resulting from any real-world beamforming implementation may degrade the intended user signal by allowing interference through these lobes or by canceling part of the intended signal where the angle of the source of the intended signal is too close to the angular position of a null placed on an interfering signal. Furthermore, since beamforming and nulling merely optimize only power reception in a preselected direction, no benefit can be derived from other signal parameters (e.g., phase or frequency).
The general field of adaptive array processing is described in the standard text by Bernard Widrow and Samuel D. Stearns, Adaptive Signal Processing, published by Prentice-Hall in 1985.
The principle of Per-Survivor Processing (PSP) is known for scalar signal processing. PSP provides a general framework for the approximation of likelihood-based data detection (search) algorithms whenever the presence of unknown quantities prevents the precise use of the classical Viterbi Algorithm (VA).
Prior work of Andreas Polydoros and Riccardo Raheli in scalar Per-Survivor Processing (PSP), as is described in U.S. Pat. No. 5,432,821 issued Jul. 11, 1995 based on a patent application serial No. 985,004 filed Dec. 2, 1992, dealt only with scalar signal analysis of multipath extracted from a single antenna. (This patent is incorporated herein by reference and made a part hereof.) The prior work of Polydoros and Raheli failed to address vector processing and thus did not deal with inputs received from multiple antennas.
According to the invention, a method and a system are provided which uses vector per-survivor processing (PSP) on the outputs of an array of antennas of arbitrary geometry to jointly estimate angles of arrival of multipath components of the transmitted data sequence (i.e., the modulated data signal) while extracting an estimate of the modulated data. The system includes an antenna array of arbitrary geometry (i.e., arbitrary spacing between elements, directional, omnidirectional, etc.), each element of which has its output applied to a corresponding matched filter. The matched filter outputs in turn are sampled and applied to a vector-type PSP engine, wherein the vector PSP engine computes an estimate of the channels between the transmitter antenna and the receiver antenna elements to construct a channel estimation matrix. Angle of arrival estimates of each of the multipath components are extracted from the channel estimation matrix for the best survivor. An estimate of the transmitted data sequence is also extracted based on the best survivor. The PSP engine performs its function in the form of equations that solve for the vector estimates of complex channels, where the channel models include multipath angles of arrival.
PSP offers superior performance as compared to xe2x80x9cclassicalxe2x80x9d adaptive antenna processing architectures where the spatial aspects are processed separately from the data. Further the PSP is implemented using straightforward digital signal processing hardware and thus lends itself to less complex and less expensive systems.
PSP is particularly well suited for array processing problems because it can deal in parallel with multiple channel uncertainties and different types of channel uncertainty (including angle of arrival (AOA)) and thereby derive a more optimal solution. In addition it accommodates time varying channels (whether due to geometry, transmitter movement, interference or noise) as part of the inherent processing. Furthermore, PSP can accomplish this without the use of any training sequence, as will be hereinafter evident.
This invention was developed in the course of research leading to the Ph.D. dissertation of the present inventor entitled MODULATION CLASSIFICATION AND DATA DETECTION OF SIGNALS IN UNKNOWN ISI ENVIRONMENTS, first published Sep. 20, 1996 (Copyright Registration No. TX 4-417-419). Specifically in Section 4.2.1.1 and in Appendix D, the inventor describes the application of Per-Survivor Processing (PSP) using multiple antennas for analyzing multipath. That work is the subject of the present patent application. PSP is a radical departure from current conventional thinking on adaptive array processing. This invention is believed to be the first implementation of vector PSP in any application.
The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.